Poly(meth)acrylic acid or partially neutralized products thereof are water-soluble polymers and widely used as water-absorbing materials, thickeners, flocculants, dispersants, treating agents for paper and fibers, and the like, making good use of their hydrophilic nature. The poly(meth)acrylic acid or partially neutralized products thereof may be formed into films from their solutions by a casting process. The resultant films are excellent in oxygen gas barrier property under dry conditions. However, these films are unsuitable for packaging of food containing a great amount of water because they show strong hydrophilic nature, and are hence markedly impaired in oxygen gas barrier property under high-humidity conditions, and moreover easy to dissolve in water.
In U.S. Pat. No. 2,169,250, it is described to polymerize a methacrylic acid monomer in an aqueous solution of polyvinyl alcohol (PVA), cast the resulting reaction mixture on a support, evaporate the water, and then heat the dry film for 5 minutes at 140.degree. C., thereby reacting PVA with polymethacrylic acid to obtain a water-insoluble film (Example I). According to the results of an investigation by the present inventors, however, any film exhibiting excellent oxygen gas barrier property under high-humidity conditions can not be obtained by such heat treatment conditions.
On the other hand, films formed of starch are excellent in oil resistance and oxygen gas barrier property, but involve a disadvantage that they are poor in mechanical strength and water resistance. Starch is a natural polysaccharide derived from plants and is composed of straight-chain amylose in which glucose units are linked by .alpha.(1-4) glycosidic bonds, and a high molecular weight amylopectin in which a great number of short amylose units are linked in a branched structure through .alpha.(1-6) glycosidic bonds. Examples of the starch include crude starch and besides, various kinds of modified starch, such as physically modified starch such as separated and purified amylose, modified starch improved in solubility in cold water by an acid, heating, an enzyme or the like, and graft-modified starch obtained by graft-polymerizing a monomer such as acrylamide, acrylic acid, vinyl acetate or acrylonitrile. These kinds of starch are hydrophilic polymers like poly(meth)acrylic acid and used not only in a field of food industry, but also in wide fields as water-absorbing materials, thickeners, flocculants, dispersants, treating agents for paper and fibers, and the like, making good use of their hydrophilic nature. Those having excellent water solubility among these kinds of starch can easily be formed into films from aqueous solutions thereof by a casting process. However, these films are unsuitable for packaging of food containing a great amount of water because they show strong hydrophilic nature, and are hence markedly impaired in oxygen gas barrier property under high-humidity conditions.
Some proposals have recently been made for producing films or sheets from mixtures of the starch and various thermoplastic resins. For example, there have been proposed biodegradable molded laminates in which a thin layer of a saponified product of an ethylene-vinyl acetate copolymer is provided on at least one side of a product molded from a mixture of a thermoplastic resin such as polyethylene and a starch polymer, such as a film (Japanese Patent Application Laid-Open No. 90339/1992), biodegradable films formed from a mixture of a PVA polymer and starch (Japanese Patent Application Laid-Open Nos. 100913/1992 and 114044/1992), water-resistant compositions composed of a PVA resin and a polysaccharide and films formed from such a composition (Japanese Patent Application Laid-Open No. 114043/1992), and sheets or films formed from a composition of a saponified product of an ethylene-vinyl acetate copolymer and starch, or laminates thereof (Japanese Patent Application Laid-Open Nos. 132748/1992, 93092/1993 and 92507/1993. However, these films are still insufficient in water resistance or oxygen gas barrier property under high-humidity conditions.